Many-body perturbation theory applied to electron pair correlation energies. II. Closed-shell second-row diatomic hydrides

Diagrammatic many-body perturbation theory (MBPT) has been applied to the correlation energy of NaH, AlH, and HCl, within a basis of 47 Slater-type functions located on both centers in the molecule. By summing all second and third order two-body diagrams plus a selection of higher order diagrams via a denominator shift, about 80% of the total correlation energy could be obtained in each case. The individual pair correlation contributions are analyzed and grouped into the shell structure of these molecules, showing that the principal deficiency lies in the inadequacy of the basis sets to describe the inner-shell correlation to extremely high accuracy. Using experimental values for the correlation energy of the Ne-like inner cores of these molecules, together with the calculated MBPT valence shell values, gives an improved estimate of the total correlation energy that is within ∼5% of the experimental result for each of the three molecules.